First of all- first impressions. The Instinct 1.5 looks a heck of a lot better than the original Instinct. I don't know if it's true but I read somewhere that when Altra first started off they didn't have enough money to hire a designer, and their first generation of shoes was not pretty. They were pretty awesome shoes though, and enough people could see past their aesthetics that they sold well, and now have a designer who can make the shoes look good.
The other big thing with Altra shoes is the width of the forefoot- if you struggle with your forefoot being squeezed, you really should take a look at Altra shoes. I've worn a lot of different shoes over the years, and most of them fall somewhere between "a bit too tight across the forefoot but I can live with it" to "Good Lord, I can't wait to take these shoes off." That's not the case with these shoes at all- they have all the room I need across the my forefoot, without feeling sloppy or "clown shoeish."
The feel underfoot is quite firm. There's not a lot of cush like you might feel with some other lightweight shoes such as the Saucony Kinvara, but there's enough cushion that I don't find the ride harsh or jarring. Another thing I noticed about the feel is that it feels very stable- not in a corrective way (like shoes that have built in stability for over pronation)- but the feeling of them being low and wide just make you feel solid on the ground. I wore my original Instincts on trails quite a few times and they were brat- of course the grip isn't the same as a trail shoe, but the stability was there and I felt very confident and comfortable on the trails.
All in all I would highly recommend the Altra Instinct 1.5. It's a great shoe- very comfortable, lots of room, and a good combination of light weight yet still with a bit of cushioning. I plan on using these for the Boston Marathon in two weeks.
If you are comfortable with low or zero drop shoes, the Altra Instinct 1.5 is worth considering. I have a feeling the Intinct platform will be my trainer of choice for years to come.